Can I send messages with attachments from the macOS app?

Yes, you can send attachments, but only in some instant messengers. 
At the moment, attachments are supported by the following sending modes:

• iMessage/SMS
• Telegram
• WhatsApp*

*WhatsApp supports only the following file formats: png, jpg, jpeg, pdf, gif, psd, webp, tiff, ico, bmp. Note that WhatsApp converts PDF files from documents to images based on the first page of the document.

Starting with macOS 14 Sonoma, attachments are no longer supported for the listed instant messengers. In macOS 13 Ventura and lower, it still works.

How to choose files for attachments?

In the new message window, choose a sending mode that supports attachments. After that, you will have “Add attachments” button. By clicking on it, you can choose which file you want to attach to the message.